The present invention relates generally to limiter circuits used to limit the amplitude of an amplitude modulated carrier signal and is particularly directed to limiter circuits using non-reactive components.
Limiter circuits are commonly used for amplitude limiting an amplitude modulated carrier signal prior to applying the signal to a variety of networks such as phase lock loops and carrier exaltation circuits. Conventional limiters characteristically exhibit a variety of reactive effects such as internal parasitic reactive effects brought about largely by the variable capacity of integrated circuit components as a function of input current, and voltage level and frequency. Additional reactive effects result from externally connected capacitors and inductors which form resonant circuits having Q's that vary with input signal level and the frequency response of which may be unsymmetrical in relation to the applied carrier. These reactive effects contribute to impart angle modulation to the limited signal, the amount of angle modulation introduced generally being a function of the instantaneous level of the applied amplitude modulated carrier. Such angle modulation can seriously degrade circuit performance especially in the case where the limiter supplies a network whose output is processed with another carrier having angle modulation information components.
For example, as disclosed in co-pending application Ser. No. 378,569 filed May 17, 1982, in television receivers using intercarrier sound detection techniques, a phase lock loop or carrier exaltation circuit may be used to generate a substantially unmodulated carrier signal having a frequency equal to the video intermediate frequency of 45.75 MHz. The unmodulated 45.75 MHz carrier may be mixed with the frequency modulated 41.25 MHz intermediate frequency audio signal to produce a 4.5 MHz intercarrier signal which is then subsequently demodulated by an FM demodulator for reproduction by a suitable speaker system. In this application, any angle modulation imparted to the 45.75 MHz video carrier generated by the phase lock loop, such as by the limiter used in association therewith, will be transferred to the intercarrier sound signal by the mixer. Upon detection and reproduction of the intercarrier signal, the imparted angle modulation components may be manifested by an audible buzz in the reproduced audio signal. Audible buzz generated in this manner is especially objectionable in high fidelity television audio transmissions such as those using stereophonic techniques.
It is therefore a basic object of the present invention to provide an improved circuit for limiting the amplitude of an amplitude modulated signal.
It is a more specific object of the invention to provide a circuit for limiting the amplitude of an amplitude modulated signal without introducing angle modulation in the limited signal.
It is yet a further object of the invention to provide a limiter circuit using no reactive components for limiting the amplitude of an amplitude modulated signal without introducing angle modulation in the limited signal.